1. Field of the Invention and Related Art Statement
The present invention relates to an apparatus for supporting an optical system, and more particularly relates to an apparatus for supporting an objective lens of an optical head movably in a focusing direction parallel to an optical axis of the objective lens as well as in a tracking direction perpendicular to the optical axis to effect focusing control and tracking control in an apparatus for writing information on an optical record disk and/or reading information out of the optical record disk.
There have been proposed various optical heads comprising an objective lens for projecting a light spot on an optical record disk, a supporting mechanism for supporting the objective lens movably in focusing and tracking directions, and a driving device for moving the objective lens in said focusing and tracking directions.
For instance, Japanese Patent Application Laid-open Publication Kokai Sho 63-195834 discloses a known optical head. FIG. 1 is a perspective view showing this known optical head. An objective lens L is secured to a lens holder H and the lens holder is fixed to a movable member M. The movable member M is movably supported by four resilient wire-like supporting members 40. The construction of these supporting wires 40 is the same, so that the construction of only one supporting member will be explained. As illustrated in FIG. 2, the supporting member 40 comprises two end portions 41, a rectilinear connecting portion 43 for connecting the end portions to each other and flexure portions 42 provided near the end portions 41. The supporting member 40 is formed by etching a metal plate such as a copper plate and a stainless steel plate. To the flexure portions 42 are secured vibration suppressing members 47, respectively. The vibration suppressing member 47 may be formed by a rubber sheet. In FIG. 2 a right hand vibration suppressing member is omitted in order to show the construction of the flexure portion 42 clearly. The flexure portion 42 comprises a ring-shaped element 44, a substantially U-shaped element 46 provided at the center of the ring-shaped element and a straight element 45 which extends within a space of the U-shaped element. It should be noted that the U-shaped element 46 and straight element 45 are connected to each other only by means of the ring-shaped element 44.
When the supporting member 40 is stretched in its longitudinal direction, the supporting member is deformed from the condition shown in FIG. 3A into the condition illustrated in FIG. 3B. That is to say, the ring-shaped element 44 is resiliently deformed into an elliptical shape and the straight element 46 is shifted in the longitudinal direction by a distance .DELTA.x. The U-shaped element 46 is not deformed.
When the objective lens L is moved in the tracking direction X, the supporting member 40 is rotated in a plane parallel to the plane of the drawing of FIG. 3C. Then, a lower half portion of the ring-shaped element 44 is compressed and an upper half portion of the ring-shaped element is stretched, and the straight element 45 is tilted within the U-shaped element 46.
When the objective lens L is moved in the focusing direction Z, the supporting member 40 is deformed as depicted in FIG. 3D. That is to say, the straight element 45 is protruded from the plane of the U-shaped element 46.
As explained above, in the known supporting member, when the objective lens L is moved, the flexure portion 42 is deformed in accordance with the direction of the movement of the objective lens. Since the vibration suppressing member 47 is secured to the flexure portion 42, vibrations in all directions are suppressed.
In general, in the optical system supporting apparatus, a center point to which a driving force is applied should be made coincident with a center of gravity of the movable member M. If these two centers are not coincident with each other, there is produced a moment about the center of gravity of the movable member M and the movable member might be rotated. Then the objective lens L might be tilted and its optical property might be deteriorated. Further the position of the objective lens L might be shifted and the information could not be read or written correctly.
In the known objective lens supporting apparatus shown in FIG. 1, when the movable member M is driven in the tracking direction X, the movable member is twisted about the Y axis to generate a rolling resonance due to the deviation of the center of gravity of the movable member with respect to the center of the driving force. This deviation in the center points is generally produced by positional errors of the moving coils, yokes and magnets and uneven distribution of magnetic flux density within magnetic gaps. The rolling movement is also produced by forces generated in the Z direction (focusing direction) at portions of tracking coils 48 which extend in the X direction (tracking direction). In order to suppress the rolling resonance produced by such 2-direction forces, it is necessary to suppress the twisting motion of the movable member M about the Y axis (track direction).
However, in the known objective lens supporting apparatus shown in FIGS. 1 to 3, there is not provided any means for suppressing the rolling resonance. When the supporting member 40 is subjected to the twisting force about the Y axis, the connecting portion 43 is liable to be twisted, because the size of the flexure portion 42 in the X axis direction is wider than the connecting portion 43 and the connecting portion 43 is not covered with the damping member 47 and thus the flexure portion is hardly twisted as compared with the connecting portion. It is true that the straight element 45 is rotated about the Y axis, but its rotational amount is small. Therefore, when the supporting member 40 is subjected to the twisting moment about the Y axis, the connecting portion 43 is twisted, but the flexure portion 42 and particularly the straight element 45 are hardly twisted. In other words, the damping member 47 does not effectively serve to suppress the twisting motion of the supporting member 40.
In order to suppress the twisting movement much more effectively, there has been proposed another known objective lens supporting apparatus which is described in Japanese Patent Application Laid-open Publication Kokai Sho 62-125544. This known apparatus is shown in FIG. 4. The objective lens L is held by a lens holder H and the lens holder is secured to a movable member M. The movable member M is connected to an intermediate member 49 by means of a first pair of leaf springs 51 and 52 movably in the focusing direction Z. The intermediate member 49 is further connected to posts of a base 50 by means of a second pair of leaf springs 53 and 54, so that the movable member M is movable in the tracking direction X. The first pair of leaf springs 51 and 52 are arranged in parallel with each other, but the second pair of leaf springs 53 and 54 are arranged to form an angle .alpha. therebetween. In this known apparatus, in order to suppress the twisting motion of the movable member M, the first pair of leaf springs 51, 52 have formed therein cut out portions 55 and 56 as illustrated in FIGS. 5A to 5D.
In the known apparatus shown in FIG. 4, the twisting movement can be suppressed by means of the leaf springs 51, 52 having the cut out portions 55 and 56 which are formed asymmetrically, but other undesired vibrations in the focus direction Z could not be removed. Moreover, when the supporting member is formed by a thin wire or strip, it is practically impossible to form the cut out portion therein.
In Japanese Utility Model Application Laid-open Publication Kokai Sho 61-55214, there is disclosed still another known objective lens supporting apparatus in which the resilient supporting member is connected to the movable member holding the objective lens and to the stationary member by means of cementing agent having the elasticity. In this known apparatus the positions at which the resilient supporting member is fixed to the movable member and to the stationary member are liable to be shifted and the optical axis of the objective lens is liable to be tilted when a large external vibration is applied or the movable member is shifted to a large extent, and the stable servo control could not be effected.
In Japanese Patent Application Laid-open Publication Kokai Sho 62-57930, there is described still another known objective lens supporting apparatus in which the objective lens supporting member includes a leaf spring, at least one lug-like element which is formed in the leaf spring by cutting and bending a portion of the leaf spring and a damping sheet which is applied on the leaf spring. This known supporting member has a drawback that the leaf spring can not be twisted easily about a longitudinal direction. Moreover, the width of the leaf spring is liable to be large.
In Japanese Patent Application Publication Kokai Sho 63-62924, there is described also a known optical head in which the objective lens is supported movably in the focusing and tracking directions. In this known optical head, a rigid rod-like member is provided, one end of which is secured to the objective lens supporting member and the other end of which is connected to the stationary member by means of a damping member such as a rubber block. In this known optical head, the rod-like member suppresses the movement of the objective lens, so that the objective lens is liable to be tilted. Further the rod-like member might produce undesired resonance. Moreover the rod-like member has a substantial weight, and thus the weight of the overall objective lens assembly becomes large.